The port of Georgetown, Grand Cayman, is a tender port for all cruise ships. The other ports on which the Magic calls usually have the facilities for docking. The main exception is St. Thomas, as was mentioned above. St. Thomas has two cruise ship piers, WICO (West Indies Company, also called Havensight), and Crown Bay. Crown Bay can accomodate smaller cruise ships, and I've never heard of the Magic docking there, although it seems that it could. The WICO pier can hold three ships, even the really large ones such as RCCI's Voyager-class ships, or Princess's Grand-class. This is where the Magic usually docks, as well.
However, in the high (winter) season, there are more ships in the Caribbean. Other lines who have ships in places such as Europe or Alaska during the summer re-position their ships to the Caribbean in the Winter. St. Thomas is one of the most popular ports in the area, so during the high season, there could be as many as five or six ships in port on a given day. On these days, the Magic is sometimes required to anchor in the harbor and ferry passengers to shore on smaller boats called "tenders."
This usually only happens during the winter months, and then usually about every other week or sometimes every third week. There are a couple of websites which have cruise ship schedules for St. Thomas. I usually use the
ships.vi site. The schedule is usually posted during the last week of the month for the following month. However, the historical schedules are still there, so you can look back through and see how often the Magic docks and how often it had to tender. This can be a fairly good indication of whether or not you will have to tender for your cruise. Again, depending on which time of year you plan on cruising, you may not have to tender. For example, the Magic is scheduled to tender on January 12 and 26 this month, but if you look at last August, it docked at WICO both times.
My personal feeling is that it's not really a big deal, and I wouldn't make an itinerary decision based on whether or not I have to tender. Someone who needs a wheelchair to get around, however, would likely have a different point of view (although I believe they can accomodate these, too). Given the two itineraries for the Magic, you would definitely have to tender once on the western, and possibly have to tender once on the eastern, depending on the time of year.